首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   3729篇
  免费   60篇
  国内免费   42篇
航空   1768篇
航天技术   1348篇
综合类   71篇
航天   644篇
  2022年   21篇
  2021年   39篇
  2018年   55篇
  2017年   62篇
  2016年   41篇
  2015年   35篇
  2014年   78篇
  2013年   103篇
  2012年   86篇
  2011年   165篇
  2010年   116篇
  2009年   143篇
  2008年   208篇
  2007年   115篇
  2006年   109篇
  2005年   118篇
  2004年   81篇
  2003年   117篇
  2002年   90篇
  2001年   106篇
  2000年   89篇
  1999年   102篇
  1998年   107篇
  1997年   102篇
  1996年   91篇
  1995年   105篇
  1994年   107篇
  1993年   64篇
  1992年   75篇
  1991年   39篇
  1990年   44篇
  1989年   74篇
  1988年   39篇
  1987年   33篇
  1986年   34篇
  1985年   114篇
  1984年   80篇
  1983年   80篇
  1982年   60篇
  1981年   104篇
  1980年   25篇
  1979年   27篇
  1978年   33篇
  1977年   25篇
  1976年   24篇
  1975年   39篇
  1972年   33篇
  1971年   25篇
  1969年   22篇
  1967年   23篇
排序方式: 共有3831条查询结果,搜索用时 0 毫秒
81.
82.
Cross-correlation properties of algebraically constructed Costasarrays   总被引:1,自引:0,他引:1  
The problem of determining the cross-correlation properties of signals based on algebraically constructed Costas arrays is addressed by examining the discrete cross-correlation of the algebraically constructed Costas arrays for a given construction and dimension. Finding two arrays that minimally correlate implies that the signals based on these arrays also minimally correlate. The properties of finite fields are reviewed, and the major algebraic constructions for Costas arrays are presented, i.e. the Welch construction and the Golomb construction. The discrete cross-correlation properties of the Costas arrays are derived for arrays of the same dimension derived from the same construction. The use of Costas arrays in the signal design problem is discussed, and examples are given to show the cross-correlation of the signals based on the algebraically constructed arrays  相似文献   
83.
The proposed KAAD (knowledge-based automated air defense) system demonstrates a man-machine environment for airspace defense systems. When the unknown aircraft is hostile, a threat rating and response methods are generated by the system. It serves as a double-check decision-making system for a war control center. In addition to this application, the KAAD system can also be a useful tool as a training program for the war controller. The capabilities of the system are limited due to the shortage of knowledge resources. It requires communications among war controllers and air fighter pilots to organize a practical knowledge base. It is shown that the KAAD system can be combined with an automated ATC (air traffic control) system to become a practical system for air defense applications  相似文献   
84.
The current status of the theoretical methods for producing the relevant atomic data is surveyed.Proceedings of the Conference Solar Physics from Space, held at at the Swiss Federal Institute of Technology Zurich (ETHZ), 11–14 November 1980.  相似文献   
85.
86.
It is shown that the volume under the wideband ambiguity function is close to the square of the signal energy. The ambiguity volume is asymptotically conserved as the signal approaches the narrowband case. The narrowband ambiguity volume is a lower bound for the volume of the wideband ambiguity function.  相似文献   
87.
The tensor characteristics of the inertial fields created by acceleration and rotation, and the gravitational fields created by masses are discussed. Although it is sometimes thought that it is impossible to distinguish between gravitational and inertial effects because of Einstein's principle of equivalence, these effects do have different, detectable tensor characteristics. The principle of equivalence is only strictly applicable at a point, while the instruments to measure these tensor fields exist over a finite region. The inertial field created by acceleration is a uniform vector field and has no gradients, while the inertial field created by rotation has a uniform cylindrically symmetric tensor gradient but none of higher order. The gravitational field created by a mass is highly nonuniform with essentially no limit to the number of higher order gradients. These differences make it theoretically possible to independently measure gravitation, rotation, and acceleration effects; to do so, some form of differential force sensor with tensor response characteristics must be used. The standard technique is static, using differential accelerometers to sense the spatial gradient characteristics of the fields. A more promising technique is dynamic; by rotation of the differential sensor, the static spatial variations are transformed into temporal variations with various frequency components. It is then possible to distinguish between the various fields by frequency filtering.  相似文献   
88.
89.
Recent observations of the energetic particles produced in solar flares indicate that the production of electrons, with energies up to about 100 keV, is a fairly common feature of small flares. In those flares the acceleration of protons and other nuclei does not extend beyond about 1 MeV.The X-ray emission often exhibits two distinct components of which the first one is produced by non-thermal, the second by thermal electrons through bremsstrahlung collisions with the ambient ions. Along with these X rays, radio emission, in the microwave region, is observed. This radio emission is usually interpreted as due to gyrosynchrotron radiation from the same electrons.In this review a discussion is presented of the processes occurring in solar flares with special reference to the acceleration and radiation processes.  相似文献   
90.
The services available for scientific ballooning are described with special emphasis on the facilities of the NCAR Balloon Flight Station at Palestine, Texas. The preparations for a typical launching, the operations during the launch, the flight and the recovery are described for illustrative purposes. Considerations on the future development of scientific ballooning are given; technological problems for extended flights are considered to be less serious than the political problems.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号